A loss of joint flexibility is experienced by individuals recovering from neuromuscular diseases, joint replacements, burns, and traumatic injuries such as bone fractures and tendon and ligament tears. In order to regain joint flexibility, it is helpful to flex or extend the joint in a repeated, controlled and quantifiable manner. It is also sometimes necessary to apply a relatively small force of a long duration or repeatedly.
Other individuals require partial immobilization to protect a traumatized joint. The partial immobilization ideally would allow the joint to move through a predetermined range of motion that is necessary for limited locomotion but not through a range of motion that is injurious to the joint, that further traumatizes the joint, or that causes pain to the individual.
Limb devices can be classified into four distinct types. One type is a "dynamic splint" in which an undirection force is applied to the joint to lengthen or shorten connective tissue. The force has typically been applied by a simple spring in the prior art. A second type of device is a brace which is used to stabilize an unstable joint. Braces may or may not include stops to limit the degree of joint flexion or extension. A third type of device is a "CPM" device or "continuous passive motion" device which applies a cyclical force to a joint for rehabilitation after procedures such as total joint replacement, joint surgery and severe burns. The cyclical force may be applied using a lead screw mechanism operated with a controller or other suitable mechanisms. A fourth type of device is an exercise device in which an undirectional force is applied to resist muscular contraction by the patient in order to increase the patient's strength in a given range of joint movement. Again, the force is typically produced by one or more springs. It is important in each of these devices that the apparatus accommodate the natural motion of the joint to ensure stability and biomechanical alignment and to prevent migration or chafing of the device on the limb. It is often the case that different devices are used on the same joint at different stages of treatment.
The major hinge joints of the limbs, namely the knee and elbow, are particularly difficult to brace and to flex effectively. This is because both the knee and elbow joints have unusual geometries. Rather than merely bending about a pivot axis, the joints are such that the lower limb pivots about a moving axis and also twists slightly. The moving axis effect can be appreciated by reference to FIG. 1, which shows in schematic form the bone structure of the knee joint. The lower femur 22 acts as a ball joint to receive the upper tibia 24. However, the ball of the lower femur is not truly spherical. Instead, it has a radius 26 that is shorter when the joint is bent as compared to a radius 28 when the joint is straightened. In addition, the ball is offset to the rear of the femur relative to the longitudinal axis of the femur. Therefore, the tibia 24 moves away from the longitudinal axis 31 of the femur 22 as the joint bends. This movement of the tibia away from the femur actually lengthens the leg as it bends. A similar process takes place at the elbow.
It can be appreciated from FIG. 1 that the tibia 24 not only moves away from the femur as the joint is bent, thereby increasing the radius of rotation, but also that the axis of rotation moves as the tibia 24 slides down the ball of the femur 22. Again, a similar process takes place at the elbow. The moving axis of rotation describes a curved line 32 which anatomists describe as an "evolute". For purposes of the present patent, an "evolute line" is defined as a curved line having a gradually decreasing radius of curvature along the line.
Another unusual aspect of the bending of a knee or elbow is that as the lower limb straightens relative to the upper limb, it simultaneously rotates along the lower limb axis. The tibia rotates so that the toes turn outward and the heels turn inward as the joint unbends. The tibia rotates in the opposite direction as the joint is bent so that the toes turn inward and the heels turn outward. Similarly, at the elbow, the radius and ulna rotate so that the palm turns downward as the elbow is unbent, and the radius and ulna rotate in the opposite directly so that the palm turns upward as the elbow is bent.
It can be appreciated that the combination of an upper limb ball that is offset from the upper limb longitudinal axis, an upper limb ball that is nonspherical, and a twisting action of the lower limb as it bends in relation to the upper limb, results in a complex movement. This complex movement has proven to be difficult to match in devices designed to brace or cyclically flex the joint.
Typical of the prior art are U.S. Pat. Nos. 4,508,111; 4,397,308; 4,485,808 and 4,538,600, all by Hepburn. These devices are designed to bias the direction in which the joint moves by stretching or increasing the available joint extension. In other words, they are dynamic splints in the terminology used herein. They generally comprise upper and lower struts which attach to the limbs of the desired joint using an appropriate attachment means. The upper and lower struts are pivotally attached to one another at the ends adjacent the joint. The pivotal attachment includes a cylindrical housing with a cam, wherein one of the struts is attached to the cam and the other bears on the cam surface through a bearing spring. Flexing or extending the joint causes a corresponding approximation or alignment of the struts relative to one another and a compression or expansion of the spring. The use of the spring allows a somewhat quantifiable and adjustable constant force to be applied to urge the flexing or extending of the joint. It can be appreciated that the devices in the Hepburn patents make no attempt to match the curved evolute line defined by the bending joint or to account for the rotation of the lower limb about its longitudinal axis as the joint bends.
Other prior art devices are in U.S. Pat. No. 4,489,718 by Martin; 4,372,298 by Lerman; 4,361,142 by Lewis; 4,487,200 by Feanny; 4,686,969 by Scott; 4,256,097 by Willis and 5,016,514 by Grood. These devices do not provide for the range of movement provided for in the present device. In addition, they generally do not include a mechanism to cyclically flex and extend the joint by application of a force in the manner of the dynamic splint embodiment of the present invention and as further described in U.S. application Ser. No. 07/495,044, of which the present is a continuation-in-part.